How do I find the manual for this and other GOG games? They used to be included in the download, but now I can't find any mention of them. Is GOG still including manuals? If not, is there some other way to learn how to play these games other than trial and error? Thanks.

Yes, manuals are still included, but sometime devs just do not provide them and people need to look for info on game wiki or on forum.

On the Gog download page you can hit More -> GOG Download links and see 6M of Goofies there, but the link is dead. Gog will have to fix this.

Are you still talking about Sunless Sea? If yes then there is NOTHING to fix as nothing is broken.

These 6MB of goodies - it is the link for GOG Downloader - https://www.gog.com/downloader - and it works ONLY if you have GOG Downloader already installed. If you don't then just use "normal" links.

GOG Downloader is an older GOG's app (now mostly replaced by Galaxy) but it still working in many cases if somebody needs just it.